A radiographic phosphor panel contains a layer of phosphor, a crystalline material which responds to X-radiation on an image-wise basis. Like many other crystalline materials, radiographic phosphors have a crystal matrix which allows for the replacement of some atoms by other similar atoms, but does not readily accept other atoms or moieties. Radiographic phosphor panels can be classified, based upon their phosphors, as prompt emission panels and image storage panels.
Intensifying screens are the most common prompt emission panels. Intensifying panels are used to generate visible light upon exposure of the intensifying panel to X-radiation. A sheet of photographic film is positioned to intercept the visible light generated and commonly is pressed against the intensifying panel within a light-tight cassette. Other prompt emission panels operate similarly, but in place of the photographic film have some other means for visualizing the X-radiation.
Storage panels have storage phosphors, that have the capability of storing latent X-ray images for later release, apparently by locally trapping electron-hole pairs created by incident X-rays. Storage phosphors are distinguishable from the phosphors used in X-ray intensifying or conversion screens. In the latter, a latent image is not stored and X-radiation causes the immediate release of visible light from irradiated phosphor crystals.
Radiation image storage panels are used in computed radiography. The panel is first exposed to X-radiation to create a latent image. The panel is then stimulated with longer wavelength radiation, resulting in the emission of radiation at a third wavelength. Typically a laser having a red or infrared beam is scanned over the panel, resulting in the emission of green or blue radiation. The emitted light is collected and the resulting signal is processed electronically to produce a final image.
Alkaline earth metal fluoro-iodide storage phosphors are described in a number of patent publications. European Patent Publication No. 0 142 734 A1 teaches a phosphor described by the formula: EQU BaF(Br.sub.1-x I.sub.x): yEu.sup.2+
This application states that ". . . the phosphor containing three kinds of elements of fluorine, bromine and iodine as halogen which is a host component of the phosphor is prominently enhanced in the luminance of stimulated emission. The radiation image recording and reproducing method employing said stimulable phosphor can be remarkably enhanced in the sensitivity."
As a result of the incorporation of iodide into the storage phosphor, problems arose regarding the stability of the phosphor. This problem has been encountered in other halide containing phosphors as well and several strategies have been developed to solve this problem.
Radiation image storage panels, unlike intensifying screens, are subject to degradative losses of both emitted light and stimulating radiation. Since these effects are cumulative, discoloration can be an even more serious issue in storage panels than in intensifying screens.
Yellowing of a phosphor layer of a radiation image storage phosphor panel, in which the phosphor contains iodine, is described in European Patent Publication No. 0 234 385 B1. The yellowing is ascribed to liberation of free iodine. The solution taught for the yellowing problem, is incorporation in the phosphor layer of a compound containing a free epoxy group and/or a compound selected from: phosphites, organotin compounds, and metal salts of organic acids, specifically: octylic acid, lauric acid, stearic acid, oleic acid, ricinoleic acid naphthenic acid, 2-ethylhexanoic acid, resin acid, synthetic carboxylic acid, benzoic acid, salicylic acid, organic phosphinous acid, phenol, and alkylphenol.
The majority of the strategies developed have involved the inclusion of additives or "stabilizers" into the image storage panels. U.S. Pat. No. 4,374,905, to Rabatin, teaches the addition of anhydrous dessicating salts to intensifying screens. In GB 2 017 140 A, intensifying screens were stabilized against discolouration and hydrolysis by incorporation of a compound containing a free epoxy group and, optionally, a dialkyl tin compound. European Patent Specification No. 0 234 385 B1 discloses a solution to the yellowing problem found in image storage panels whose phosphors contain iodine. The solution involves incorporating into one of the layers of the panel a compound containing a free epoxy group and/or a compound selected from: phosphites, organotin compounds, and metal salts of particular organic acids. None of the above prior art describes chemical or compositional alterations on the phosphor itself.
U.S. Pat. No. 3,617,332, to Lehmann, discloses a method for stabilizing finely divided phosphors. An alkaline earth metal sulfide phosphor, such as CaS:Ce, is exposed to a solution of ammonium fluoride which reacts with the surface portion of the phosphor. The phosphor is then said to be "stabilized" with respect to water and air. This patent states:
"When the phosphor is mixed in the ammonium fluoride solution, the surface portions of the phosphor react with the fluorine to form a thin protecting film of calcium fluoride, although this film is not thick enough to inhibit the luminescent properties of the prepared phosphor".
No loss (or gain) in phosphor efficiency (speed) is observed as a result of the method. The "thin protecting film" of calcium fluoride is not itself luminescent.
It would be desirable to provide improved radiation image storage panels with high sensitivity and stability against yellowing and/or hydrolysis.